Author's Comments:See also commentaries by seven other
researchers in this issue.

Abstract:A popular view of interval timing in animals
is that it is driven by a discrete
pacemaker-accumulator (PA) mechanism that
yields a linear scale for encoded time. But
PA mechanisms are fundamentally at odds with
the Weber-law property of interval timing and
experiments supporting linear encoded time
can be interpreted in other ways. We argue
that the dominant PA theory, scalar
expectancy theory (SET), fails to explain
some basic properties of operant behavior on
interval-timing procedures and can only
accommodate a number of discrepancies by
modifications and elaborations that raise
questions about the entire theory. We
propose an alternative that is based on
principles of memory dynamics derived from
the multiple-time-scale (MTS) model of
habituation. The MTS timing model can
account for data from a wide variety of
time-related experiments: proportional and
Weber-law temporal discrimination, transient
as well as persistent effects of
reinforcement omission and reinforcement
magnitude, bisection, the discrimination of
relative as well as absolute duration, the
choose-short effect and its analogue in
number-discrimination experiments.
Resemblances between timing and counting are
an automatic consequence of the model. We
also argue that the transient and persistent
effects of drugs on time estimates can be
interpreted as well within MTS theory as in
SET. Recent real-time physiological data
conform in surprising detail to the
assumptions of the MTS habituation model.
Comparisons between the two views suggest a
number of novel experiments.